By way of non-limiting example, dynamic solenoid-operated power-off brakes are used in horizontal stabilizer electric motor units (HSEMU) responsible for actuating control surfaces of an aircraft horizontal stabilizer. A known design used for this purpose is shown in FIG. 1. A motor-driven shaft carries a rotor plate mounted on a splined end of the shaft for rotation with the shaft about a longitudinal axis of the shaft. The brake comprises a solenoid including coils and an armature. A first brake disk is arranged on the armature facing one side of the rotor plate. A second brake disk is arranged on a backing plate facing an opposite side of the rotor plate. When power is supplied to the solenoid coil, the armature with the first brake disk is attracted toward the coils (to the left in FIG. 1) against the urging of springs (only one spring being visible in the cross-sectional view of FIG. 1). Thus, in the power ON state, the rotor plate is not frictionally engaged by the brake disks, thereby allowing rotation of the rotor plate and shaft. When power to the solenoid coils is turned OFF, the de-energized coils no longer pull the armature to the left, such that the springs can force the armature axially toward the rotor plate. When this happens, the brake disks are brought into frictional engagement with opposite sides of the rotor plate to frictionally brake rotation of the rotor plate and shaft.
As may be understood, the prior art brake design shown in FIG. 1 is very sensitive to wearing of the brake disks. As the brake disk pads wear and become thinner, the air gap between the armature and coils of the solenoid becomes greater, thereby reducing the ability of the coils to pull in the armature against the bias of the springs. Maintenance is required to keep the brake mechanism working properly.
There is a need for a dynamic solenoid-operated power-off brake that does not experience decreased performance due to wear.